Interdigital stripline filter means with thin shorting shim

ABSTRACT

A stripline interdigital filter means in which the stripline ground planes are metal layers on the outer surfaces of a pair of mating sheets of an insulating material, the resonator strips and coupling strips of the filter are a printed circuit on the mating surfaces of one of the insulating sheets, and the shorting member of the filter is a thin metal shim having shorting tabs extending between the mating surfaces of the insulating sheets and clamped against the resonator strips.

United States Patent [72] Inventor Michael Moore 3,113,277 12/1963 Casper et a1. 333/84(M)X Philadelphia, Pa. 3.258,?24 6/1966 Walsh et a1. 333/84( M) [2]] Appl. No. 757,529 3,309,629 3/1967 Hessler, Jr. 333/84X [22] Filed Sept. 5,1968 3,327,255 6/1967 Bolljahn, et a1. 333/84X Palemed y 13,197! 3,374,537 3/1968 Doelp, Jr. 317/101(CP) [73] Assignee American Electronic Laboratories, Inc. 3,379,938 4/1968 Bywaters 317/10l(CP) o an Pa. 3,381,372 5/1968 Capano 317/101(CP) 3,470,483 9/1969 Wagner, et a1 333/ 84X [54] lNTERDlGlTAL STRIPLINE FILTER MEANS WITH THIN SHORTING SHlM 10 Claims, 7 Drawing Figs. [52] U.S. Cl 333/73, 333/84, 317/101, 339/I9(Request), 29/600( Request), 29/629(Request) [51] Int. Cl 1101p 3/08, HOlp 11/00, H03h 13/00 [50] Field of Search 333/73 (S),

84 (M); 317/101 (CC), 101 (CP); 333/73, 84, 3,000 (lnquired); 339/ 1 9 (Cursory) [56] References Cited UNITED STATES PATENTS 2,937,347 5/1960 Matthaei et a1 333/84(M)X Primary Examiner-H. K. Saalbach Assistant Examiner-Wm. H. Punter Attorney-Jacob Trachtman ABSTRACT: A stripline interdigital filter means in which the stripline ground planes are metal layers on the outer surfaces of a pair of mating sheets of an insulating material, the resonator strips and coupling strips of the filter are a printed circuit on the mating surfaces of one of the insulating sheets, and the shorting member of the filter is a thin metal shim having shorting tabs extending between the mating surfaces of the insulating sheets and clamped against the resonator strips.

SHEET 1 [IF 2 PATENTEI] m I 81971 INVE/VTQ/P MICHAEL MOORE lNTlEEtDllGllTAL STRMPMNE FELTER MEANS WITH THIN SHORTING SllilM The invention relates to electrical filter means and particularly stripline filter means employing printed circuits.

lnterdigital filters are, in general, constructed of an array of metal stubs laid in spaced apart, side-byside relation with alternate stubs being connected together by shorting bars extending across the ends of the connected stubs. This provides two sets of interdigited short-circuited stubs. l-leretofore such filters have been constructed of machined, pressed or punched rods which are assembled together in the proper array. Since the electrical characteristics of such a filter are a function of the size and spacing between the interdigited stubs, the accuracy to which the stubs are formed and assembled is critical in order to achieve a filter having desired electrical characteristics. Since such filters have been formed of many parts which must be formed and assembled with great accuracy, the making of such filters has been a time consuming and expensive operation. Therefore, it is a principal object of the invention to provide a stripline interdigital filter construction formed of a minimum number of parts which can be easily and quickly assembled even by an unskilled person, and will still provide the desired electrical characteristics. I

In order to achieve the above, as well as many more objects, the invention provides a stripline interdigital filter comprising a pair of metal housing bodies secured together and having opposed, mating surfaces. The housing bodies have opposed mating cavities in their mating surfaces. A separate stripline element is seated in each of the cavities. Each of the stripline elements comprises a sheet of insulating material having a metal film extending over and secured to the surface of the insulating sheet facing the bottom of the cavity. The other surface of the insulating sheet of one of the stripline elements has thereon a printed circuit interdigited array of filter resonator strips and a pair of coupling strips. A thin metal shorting shim is clamped between the mating surfaces of the housing bodies and extends around the stripline elements. The shorting shim has a plurality of tabs extending between the mating surfaces of the stripline elements with each of the tabs being clamped against a separate one of the resonator strips. A pair of coaxial conductors are mounted on at least one of the housing bodies. Each of the conductors has a conductor pin extending into contact with a separate one of the coupling strips on the stripline element.

For the purpose of illustrating the invention there is shown in the drawings forms which are presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

lFlG. l is a perspective view of the stripline filter means of the present invention.

H0. 2 is an exploded perspective view of the stripline filter means of FIG. l.

H6. 3 is a sectional view taken on line 3-3 of FIG. 1.

FIG. 4i is an exploded perspective view of a modification of the stripline filter means of the present invention.

FIG. 5 is a transverse sectional view of the assembled stripline filter means of HO. 4.

MG. 45 is a front plan view of the printed-circuit element of the stripline filter means of the present invention.

FIG. 7 is a curve showing the electrical characteristics of the stripline filter means of the present invention.

Referring initially to FIGS. l-3 inclusive, the stripline filter means it) of the present invention comprises a pair of rectangular, metal housing bodies 12a and 12b. The housing bodies 120 and ll2b have rectangular cavities Ma and 14b respectively in their opposed inner flat surfaces. A pair of spaced semicylindrical recesses 16a and 18a extend across the inner surface of the housing body 12a from the upper edge of the housing body to the cavity 14a, and a corresponding pair of spaced, semicylindrical recesses 16b and 18b extend across the inner surface of the housing body 12b from the upper edge thereof to the cavity Mb. The cavities Ma and 14b, recesses 16a and lob, and recesses 113a and 18b are positioned so as to be in opposed mating relation when; the housing bodies 12a and 12b are brought together. The recesses 16a, 16b, 18a and 1812 are stepped in diameter having portions of largest diameter adjacent the upper edges of the housing bodies, and portions of smallest diameter adjacent the cavities. A plurality of spaced-apart aligning pins 20 are secured to and project from the inner surface of the housing body 12a, and are adapted to fit into aligning holes 22 in the housing base 12!) to properly align the housing bases with respect to each other when they are brought together. Screws 24 are adapted to extend through holes 26b in the housing base 12b and to be threaded into threaded holes 260 in the housing base 12a to secure the housing bases together. Holes 26b are counterbored at the outer surface of the housing base ll2b so as to receive the heads of the screws 24.

A stripline and filter element 28a fits in the cavity 14a in the housing body 12a, and a stripline element 28b fits in the cavity 14b in the housing body 12b. Stripline and filter element 28a comprises a rigid, substantially rectangular sheet 300 of an electrical insulating material, such as a plastic, of a size and thickness equal to the size and depth of the cavity Ma. The corners of the sheet 300 are cut off to provide for greater ease of inserting and removing the stripline and filter element 23a from the cavity 14a. A thin layer 32a of an electrically conductive metal, such as copper, extends over and is secured to one surface of the sheet 30a. The :metal layer 32a may be suitably plated on the surface of the sheet 30a, or may be a thin metal foil bonded to the sheet 30a by a suitable cement. The metal layer 32a provides one ground plane of the stripline.

Two sets of narrow strips 34 and 36 of an electrically conductive metal, such as copper, are secured to and extend transversely across the other surface of the sheet 300 parallel to but spaced from each other. The strips 34 and 36 are arranged along the sheet 30a in alternating relation with the strips 34 extending from a'first edge of the sheet 3011 to a point short of the second opposite edge, and the strips 36 extending from the second edge to a point short of the first edge. The

strips 34 and 36 provide the resonator bars of an interdigital.

band pass filter. Input and output coupling strips 38 and 40 of an electrically conductive metal extend across the sheet 30a at respective ends of and parallel to the resonator bars 34 and 36. The coupling strips 38 and 40 extend from the first edge of the sheet 30a to a point short of the second opposite edge. The spacing between the coupling strips 38 and 40 is equal to the spacing between the conductor recesses 16a and 18a in the housing body Ma.

The resonator strips 34 and 36 and the coupling strips 38 and 40 are formed by any of the well-known techniques for forming a printed circuit. For example, a layer of the metal is applied over the entire surface of the sheet 30a either by plating the metal on the sheet or by bonding a metal foil to the sheet by a suitable cement. A resist material is coated on the surface of the metal film over the areas which will provide the resonator strips 34 and 36 and the coupling strips 33 and 40. The uncoated portions of the metal film is removed, such as by chemical etching, leaving the resonator strips and coupling strips. The resist material is then removed.

As shown in FIG. 6, the coupling strips 38 and 40 are slightly wider than the resonator strips 34 and 36, and are spaced closer to their adjacent resonator strips than the spacing between the interleaved resonator strips. The resonator strips 34 and 3b and the coupling strips 38 and 40 are made approximately one-quarter wavelength of the midband frequency. The spacing between the resonator strips is critical since the bandwidth is a function of this spacing. The spacing between the resonator strips is symmetrical about the centermost resonator strip with the spacing between the end strips being slightly less than the spacing between the center strips.

The stripline element 28b comprises a rectangular, rigid sheet 30b of an electrical insulating material of the same size and shape as the sheet 30a of stripline and filter element 23a. A thin layer 32b of an electrically conductive metal, such as copper, extends over and is secured to one surface of the sheet 36b. The metal layer 32b may be suitably plated on the sheet Mb, or may be a thin metal foil bonded to the sheet by a suitable cement. The metal layer 3217 provides the other ground plane of the stripline. The stripline elements 28a and 28b are mounted in the cavities 14a and Mb of the housing bases 12a and 12b with the ground planes 32a and 32b being seated against the bottom of the cavities.

A shorting shim $2 is mounted between the mating surfaces of the housing bodies 12a and 12b. Shorting shim 42 comprises a thin, rectangular foil of an electrically conductive metal, such as copper, of the same dimensions as the mating surfaces of the housing bodies. The shorting shim 42 has a rectangular opening 44 therethrough of the same dimensions as the cavities 14a and Mb. Holes 46 are provided through the shim 42 to permit the screws 24 to pass therethrough, and smaller alignment holes 458 are provided to receive the aligning pins 20. A pair of spaced gaps 50a and 59b extend trough the shim 42 from the upper edge thereof to the edge of the opening 44. The gaps Etta and 50!; are positioned and shaped to correspond with the conductor recesses 16a 16b and 18a and l respectively in the housing bodies 120 and 12b.

A plurality of spaced tabs 52 extend downwardly from the upper edge of the opening 44 in the shim 42, and a plurality of spaced tabs 54 extend upwardly from the lower edge of the opening 44. The number and positioning of the tabs 52 correspond to the number and positioning of the resonator strips 34 of the stripline and filter element 28a. The number and positioning of the tabs 54 correspond to the number and positioning of the resonator strips 36. Thus, when the shim 42 is clamped between the housing bodies 12a and 12b, the tabs 52 and 54 extend over the ends of their respective resonator strips 34 and 3b, and are clamped into contact with the resonator strips by the stripline element 28b.

When the housing bodies 220 and 12b are clamped together, the conductor recesses 16a and 1&1 in the housing body 12a mate with the conductor recesses 16b and 18b in the housing body i'i' bto provide a pair of cylindrical conductor openings. An electrical insulator 56 having a hole therethrough is seated in the upper portion of each of the conductor openings. A pair of coaxial conductors 53 are mounted on the upper edges of the housing bodies 120 and 12b by screws 6'1 with each of the conductors being in alignment with a separate one of the conductor openings. Each of the conductors 58 has a conductor pin 62 extending through its respective conductor opening and contacting one of the coupling strips 38 or 40 of the stripline and filter element 28a.

The printed-circuit-type of construction of the stripline and filter element 2811 of the present invention provides for greater ease of making the filter element with very high accuracy of the positioning of the resonator and coupling strips. Since the dimensionsand spacing of the resonator strips is critical in order to obtain the desired electrical characteristics of the filter, the ease with which these elements can be formed with a high degree of accuracy is an important factor in the manufacture of this type of filter. Even though the resonator strips are very thin, they provide a filter having excellent electrical characteristics as shown in FIG. 7

The printed circuit construction of the stripline and filter element 2&1 and the shorting shim construction of the present invention also provides a stripline filter which can be easily and quickly assembled even by an unskilled person. To assemble the stripline filter It) of the present invention at the stripline elements 231: and 28b are first inserted in the cavities 14a and Mb of their respective housing bodies I2a and 12b with the metal ground planes 32a and 32b facing the bottom of the cavities. The shorting shim 42 is then placed on the inner surface of the housing body 120 with the shim being properly aligned by the aligning pins 20 fitting through the alignment holes 48. The housing body 12b is then mounted on the housing body 120 with proper alignment being achieved by the aligning pins 20 fitting into the aligning holes 22. The screws 24 are inserted and tightened to secure the housing bodies 12a and 1% together. This automatically clamps the shorting tabs 52 and 54 between the stripline elements 28a and 28b and in electrical contact with the resonator strips 34 and 36. The insulators 56 are inserted in the conductor openings, and the conductors 58 are mounted on the housing bodies 12a and 12 to complete the assembly.

Referring to FIGS. 5 and 6, a modification of the stripline filter means of the present invention is generally designated as 10'. Stripline filter means 10 is of the same construction as the stripline filter means 10 of FIGS. l3 except as to the manner of mounting the coaxial conductors 58.

In the stripline filter means I0, the conductor holes 16' and 18 extend through the housing body 12b from the outer surface to the inner surface with the bottom one-half of each of the conductor holes extending horizontally directly into the cavity Mb. The stripline element 28b, which is seated in the cavity 14b in the housing body 12b, has a pair of semicylindrical notches 64a and 64b in its upper edge which are in alignment with the conductor holes 16 and 18' respectively. The shorting shim 42 has a pair of semicylindrical notches 66a and 66b in the upper edge of the opening 44 therethrough which are also in alignment with the conductor holes 16 and 18. The conductor pins 62' extend through the holes 16' and 18, the notches 64a and 64b in the stripline element 28b, and the notches 66a and 66b in the shim 42. The ends of the conductor pins 62 contact the coupling strips 38 and 40 of the stripline and filter element 28a. As in the stripline filter means 10 of FIGS. 1-3, the shorting tabs 52 and 54 of the shorting shim 44' are clamped between the stripline and filter element 28a and the stripline element 28b and are held in contact with the resonator strips 34 and 36. The construction of the stripline filter means 10 provides for greater ease of making contact between the conductor pins 62 and the coupling strips 38 and 40'. Also, it provides a completely one piece shorting shim 42' which is easier to handle during the assembling of the stripline filter means.

The present invention may be embodied in other specific 2 forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

I claim:

1. A stripline filter means comprising a pair of sheets of an electrical insulating material having inner opposed mating surfaces, conductive means extending along the outer surface of each of said sheets, said conducting means providing the ground planes of the stripline filter means, a plurality of thin narrow strips of an electrically conductive metal secured to and extending across the mating surface of one of said sheets in side-by-side spaced parallel relation, said strips providing the resonators of the stripline filter means, a pair of narrow coupling strips of an electricallyconductive metal secured to and extending across the mating surface of said one sheet parallel to said resonator strips, each of said coupling strips being at an opposite end of the array of the resonator strips, a thin shorting shim of an electrically conductive metal extending around the periphery of said sheets, said shorting shim having a plurality of spaced tabs extending between the mating surfaces of said sheets with each of said tabs contacting a separate one of said resonator strips and electrically connecting together a plurality of said resonator strips, means securing said sheets together with the shorting shim tabs being clamped between said sheets and into contact with their respective resonator strips, and a pair of conductors each electrically connected with a respective one of the coupling strips.

2. The means of claim 1 in which the shorting shim has two sets of tabs, one set of the tabs extending between said sheets from one edge of the sheets and contacting alternate resonator strips, and the other set of tabs extending between said sheets from the opposite edge of the sheets and engaging the other resonator strips.

3. The means of claim 1 in which said pair of conductors each have a conductor pin extending into contact with a respective one of said coupling strips.

4. The means of claim 1 in which said conductive means comprises first and second layers of electrically conducting metal respectively extending over and secured to the outer surface of each of said sheets.

5. The means of claim 4 in which the means securing the sheets together comprises a pair of housing bodies having opposed mating surfaces, opposed mating cavities in the mating surfaces of the housing bodies, each of said sheets being seated in a separate one of said cavities with the ground plane layer facing the bottom of the cavity, and means securing said housing bodies together.

6. The means of claim 5 in which the shorting shim is clamped between the mating surfaces of said housing bodies.

7. The means of claim 6 in which a plurality of alignment pins are secured to and extend perpendicularly from the mating surface of one of said housing bodies, the shorting shim and the mating surface of the other housing body each has alignment holes therethrough through which the alignment pins extend to align the housing bodies and the shorting shim with respect to each other.

8, The means of claim 7 in which the mating surface of each of said housing bodies has a pair of spaced conductor recesses extending from an edge of the housing body to an edge of the cavity, each of the conductor recesses in one of the housing bodies being in opposed mating relation with a separate one of the conductor recesses in the other housing body, the conductors are mounted on said edges of the housing bodies with .each conductor being in alignment with a respective set of mating conductor recesses, said conductors each having a conductor pin extending through its said conductor recess into contact with its respective coupling strip.

9. The means of claim 8 in which the shorting shim is broken along the edges of the two sets of the conductor recesses.

10. The means of claim 7 in which one of the housing bodies has a pair of spaced conductor holes extending therethrough from its outer surface and opening into the cavity in said one housing body, the sheet in the cavity of said one housing body has a pair of spaced notches in an edge thereof with each notch being in alignment with a separate one of said conductor holes, the conductors are mounted on the outer surface of said one housing body with each conductor being in alignment with a separate one of the connector holes, said conductors each having a conductor pin extending through its said conductor hole and the notch in said sheet into contact with its respective coupling strip. 

1. A stripline filter means comprising a pair of sheets of an electrical insulating material having inner opposed mating surfaces, conductive means extending along the outer surface of each of said sheets, said conducting means providing the ground planes of the stripline filter means, a plurality of thin narrow strips of an electrically conductive metal secured to and extending across the mating surface of one of said sheets in side-by-side spaced parallel relation, said strips providing the resonators of the stripline filter means, a pair of narrow coupling strips of an electrically conductive metal secured to and extending across the mating surface of said one sheet parallel to said resonator strips, each of said coupling strips being at an opposite end of the array of the resonator strips, a thin shorting shim of an electrically conductive metal extending around the periphery of said sheets, said shorting shim having a plurality of spaced tabs extending between the mating surfaces of said sheets with each of said tabs contacting a separate one of said resonator strips and electrically connecting together a plurality of said resonator strips, means securing said sheets together with the shorting shim tabs being clamped between said sheets and into contact with their respective resonator strips, and a pair of conductors each electrically connected with a respective one of the coupling strips.
 2. The means of claim 1 in which the shorting shim has two sets of tabs, one set of the tabs extending between said sheets from one edge of the sheets and contacting alternate resonator strips, and the other set of tabs extending between said sheets from the opposite edge of the sheets and engaging the other resonator strips.
 3. The means of claim 1 in which said pair of conductors each have a conductor pin extending into contact with a respective one of said coupling strips.
 4. The means of claim 1 in which said conductive means comprises first and second layers of electrically conducting metal respectively extending over and secured to the outer surface of each of said sheets.
 5. The means of claim 4 in which the means securing the sheets together comprises a pair of housing bodies having opposed mating surfaces, opposed mating cavities in the mating surfaces of The housing bodies, each of said sheets being seated in a separate one of said cavities with the ground plane layer facing the bottom of the cavity, and means securing said housing bodies together.
 6. The means of claim 5 in which the shorting shim is clamped between the mating surfaces of said housing bodies.
 7. The means of claim 6 in which a plurality of alignment pins are secured to and extend perpendicularly from the mating surface of one of said housing bodies, the shorting shim and the mating surface of the other housing body each has alignment holes therethrough through which the alignment pins extend to align the housing bodies and the shorting shim with respect to each other.
 8. The means of claim 7 in which the mating surface of each of said housing bodies has a pair of spaced conductor recesses extending from an edge of the housing body to an edge of the cavity, each of the conductor recesses in one of the housing bodies being in opposed mating relation with a separate one of the conductor recesses in the other housing body, the conductors are mounted on said edges of the housing bodies with each conductor being in alignment with a respective set of mating conductor recesses, said conductors each having a conductor pin extending through its said conductor recess into contact with its respective coupling strip.
 9. The means of claim 8 in which the shorting shim is broken along the edges of the two sets of the conductor recesses.
 10. The means of claim 7 in which one of the housing bodies has a pair of spaced conductor holes extending therethrough from its outer surface and opening into the cavity in said one housing body, the sheet in the cavity of said one housing body has a pair of spaced notches in an edge thereof with each notch being in alignment with a separate one of said conductor holes, the conductors are mounted on the outer surface of said one housing body with each conductor being in alignment with a separate one of the connector holes, said conductors each having a conductor pin extending through its said conductor hole and the notch in said sheet into contact with its respective coupling strip. 